From 68061e3470210703cb15594194718d35094afdc0 Mon Sep 17 00:00:00 2001 From: Jeff King Date: Thu, 29 Aug 2019 14:37:26 -0400 Subject: [PATCH] fast-import: disallow "feature export-marks" by default The fast-import stream command "feature export-marks=" lets the stream write marks to an arbitrary path. This may be surprising if you are running fast-import against an untrusted input (which otherwise cannot do anything except update Git objects and refs). Let's disallow the use of this feature by default, and provide a command-line option to re-enable it (you can always just use the command-line --export-marks as well, but the in-stream version provides an easy way for exporters to control the process). This is a backwards-incompatible change, since the default is flipping to the new, safer behavior. However, since the main users of the in-stream versions would be import/export-based remote helpers, and since we trust remote helpers already (which are already running arbitrary code), we'll pass the new option by default when reading a remote helper's stream. This should minimize the impact. Note that the implementation isn't totally simple, as we have to work around the fact that fast-import doesn't parse its command-line options until after it has read any "feature" lines from the stream. This is how it lets command-line options override in-stream. But in our case, it's important to parse the new --allow-unsafe-features first. There are three options for resolving this: 1. Do a separate "early" pass over the options. This is easy for us to do because there are no command-line options that allow the "unstuck" form (so there's no chance of us mistaking an argument for an option), though it does introduce a risk of incorrect parsing later (e.g,. if we convert to parse-options). 2. Move the option parsing phase back to the start of the program, but teach the stream-reading code never to override an existing value. This is tricky, because stream "feature" lines override each other (meaning we'd have to start tracking the source for every option). 3. Accept that we might parse a "feature export-marks" line that is forbidden, as long we don't _act_ on it until after we've parsed the command line options. This would, in fact, work with the current code, but only because the previous patch fixed the export-marks parser to avoid touching the filesystem. So while it works, it does carry risk of somebody getting it wrong in the future in a rather subtle and unsafe way. I've gone with option (1) here as simple, safe, and unlikely to cause regressions. This fixes CVE-2019-1348. Signed-off-by: Jeff King --- Documentation/git-fast-import.txt | 14 ++++++++++++++ fast-import.c | 25 +++++++++++++++++++++++++ t/t9300-fast-import.sh | 23 +++++++++++++++-------- transport-helper.c | 1 + 4 files changed, 55 insertions(+), 8 deletions(-) diff --git a/Documentation/git-fast-import.txt b/Documentation/git-fast-import.txt index 3d3d219e58..fbb3f914f2 100644 --- a/Documentation/git-fast-import.txt +++ b/Documentation/git-fast-import.txt @@ -50,6 +50,20 @@ OPTIONS memory used by fast-import during this run. Showing this output is currently the default, but can be disabled with --quiet. +--allow-unsafe-features:: + Many command-line options can be provided as part of the + fast-import stream itself by using the `feature` or `option` + commands. However, some of these options are unsafe (e.g., + allowing fast-import to access the filesystem outside of the + repository). These options are disabled by default, but can be + allowed by providing this option on the command line. This + currently impacts only the `feature export-marks` command. ++ + Only enable this option if you trust the program generating the + fast-import stream! This option is enabled automatically for + remote-helpers that use the `import` capability, as they are + already trusted to run their own code. + Options for Frontends ~~~~~~~~~~~~~~~~~~~~~ diff --git a/fast-import.c b/fast-import.c index d64609b083..967077ad0b 100644 --- a/fast-import.c +++ b/fast-import.c @@ -366,6 +366,7 @@ static uintmax_t next_mark; static struct strbuf new_data = STRBUF_INIT; static int seen_data_command; static int require_explicit_termination; +static int allow_unsafe_features; /* Signal handling */ static volatile sig_atomic_t checkpoint_requested; @@ -3320,6 +3321,8 @@ static int parse_one_option(const char *option) show_stats = 0; } else if (!strcmp(option, "stats")) { show_stats = 1; + } else if (!strcmp(option, "allow-unsafe-features")) { + ; /* already handled during early option parsing */ } else { return 0; } @@ -3327,6 +3330,13 @@ static int parse_one_option(const char *option) return 1; } +static void check_unsafe_feature(const char *feature, int from_stream) +{ + if (from_stream && !allow_unsafe_features) + die(_("feature '%s' forbidden in input without --allow-unsafe-features"), + feature); +} + static int parse_one_feature(const char *feature, int from_stream) { const char *arg; @@ -3338,6 +3348,7 @@ static int parse_one_feature(const char *feature, int from_stream) } else if (skip_prefix(feature, "import-marks-if-exists=", &arg)) { option_import_marks(arg, from_stream, 1); } else if (skip_prefix(feature, "export-marks=", &arg)) { + check_unsafe_feature(feature, from_stream); option_export_marks(arg); } else if (!strcmp(feature, "get-mark")) { ; /* Don't die - this feature is supported */ @@ -3464,6 +3475,20 @@ int cmd_main(int argc, const char **argv) avail_tree_table = xcalloc(avail_tree_table_sz, sizeof(struct avail_tree_content*)); marks = pool_calloc(1, sizeof(struct mark_set)); + /* + * We don't parse most options until after we've seen the set of + * "feature" lines at the start of the stream (which allows the command + * line to override stream data). But we must do an early parse of any + * command-line options that impact how we interpret the feature lines. + */ + for (i = 1; i < argc; i++) { + const char *arg = argv[i]; + if (*arg != '-' || !strcmp(arg, "--")) + break; + if (!strcmp(arg, "--allow-unsafe-features")) + allow_unsafe_features = 1; + } + global_argc = argc; global_argv = argv; diff --git a/t/t9300-fast-import.sh b/t/t9300-fast-import.sh index 1ba20c1f1a..ba5a35c32c 100755 --- a/t/t9300-fast-import.sh +++ b/t/t9300-fast-import.sh @@ -2117,6 +2117,11 @@ test_expect_success 'R: only one import-marks feature allowed per stream' ' test_must_fail git fast-import input && + test_must_fail git fast-import input <<-EOF && feature export-marks=git.marks @@ -2127,7 +2132,7 @@ test_expect_success 'R: export-marks feature results in a marks file being creat EOF - git fast-import one.marks && tail -n +3 marks.out > two.marks && - git fast-import --import-marks=one.marks --import-marks=two.marks in = helper->out; argv_array_push(&fastimport->args, "fast-import"); + argv_array_push(&fastimport->args, "--allow-unsafe-features"); argv_array_push(&fastimport->args, debug ? "--stats" : "--quiet"); if (data->bidi_import) {